--- In digitalradio@yahoogroups.com, "n0sya" <[EMAIL PROTECTED]> wrote: > > How does nbems fare in weak signal conditions compared to other modes > such as olivia and mt63? >
NBEMS is the software package, not an actual mode. It includes PSK31, PSK63, PSK 125, PSK250, MFSK16 and RTTY. MT63 and Olivia are not offered. One primary goal for this software is "high" speed message transfers on VHF and UHF where something like PSK250 can be used with good outcome. On HF, the noise level does not usually support the higher speed PSK operations but PSK31 and PSK63 do quite well on HF. The software uses ARQ ... the message is sent, the other stations sends an acknowledgment from time to time. If there was a error (due to no decode of a weak signal for example) the section of the message would be repeated until the station acknowledges receipt. Thus the transfer rate can be slow but the text/copy will be 100% if the files xfer is successful. While Olivia and MT63 are vastly superior to PSK31 under most weak signal situations, the ARQ aspects of NBEMS will make it more reliable if accuracy is what you desire. ALE 400 and RFSM-8000 offer alternatives . ALE-400 is available in Multipsk but is not widely used. RFSM is even less used and some baud rates are not legal for USA amateurs to use. I believe the authors of NBEMS had a goal of facilitating the accurate transfer of messages via methods widely used by hams in everyday application. Thus PSK and RTTY, very commonly used. While they will not perform as "well" as ALE 400, RSFM 8000, Winlink, Pactor II, III, in some circumstances, the expectation is that a PSK based NBEMS system will make up for it's lack of sophisticated modulation schemes via its accuracy and simplicity. One observation to keep in mind...when I have used NBEMS and have been receiving files via PSK31 ARQ , the received text in VBdigi (without ARQ) is almost always as good as the ARQ PSK via FLARQ. Most of the digital modes under average conditions will give you the ability to send a highly accurate message. NBEMS, ALE 400 and RSFM offer the "comfort" that it will be 100% accurate if the file transfer is completed. Thus, in theory , if that emergency message you intercept says "my latitude is 40.0446" and Olivia decodes it as 49.0046 , we have a problem. The error correction in NBEMS and ALE400 will ensure it is accurate. The dilemma: Under bad conditions you might get perhaps 70% of a message in Olivia with several errors, while under the same bad conditions you may only get 30% using PSK, ALE 400 or RSFM (hypothetical numbers) and not enough to complete the file transfer. Andy K3UK
